With the technology foundation of the Solid State Lighting, some lighting facilities have been produced for daily and landscape uses as well as high power lighting facilities for industrial uses. These lighting facilities have a somewhat lower color temperature, T≦3,500K, falling into the category of Warm White Lighting. If the color temperature is higher, T>4,600K, the semiconductor facilities will be categorized as Cold White Light.
Most modern lighting facilities are usually bulky. Semiconductor lighting facilities having small volume and high power, therefore, are of very high demand.
To overcome the aforementioned drawback, it is essential to have a special single piece high-power device in semiconductor light sources. This technology has been developed so far no more than 20 years and a Japanese engineer S, Nakanura proposed a new lighting framework based on indium nitride (InN) and gallium nitride (GaN), comprising In—Ga—N heterojunction (or P-N junction) with a large number of quantum wells in nanometer scales (please refer to the 1997 technical papers “The Blue Laser diode Sp. 1997” published by S, Nakanura; the content of this paper will not be detailed herein.).
This new development is based on a heterojunction device, Stokes inorganic phosphor powder conversion layer based on the framework of gallium nitride semiconductor framework, proposed by Russian engineers 30 years ago. The aforementioned conversion layer is the anti-Stokes phosphor powder originally used in light-emitting diodes.
In 1998, an engineer, S, Nakamura, working for the Nichia Company developed a white light-emitting diode based on the primary blue light of semiconductor from light-emitting-diodes. The blue light emission of the semiconductor heterojunction in light-emitting diode is combined with yellow light emission generated by a large amount of phosphor powder to obtain uniform white light according to the Newton Complementary Color Principle (Blue and yellow are complementary colors.). The aforementioned white light emission has been widely used in black-and-white television screen and RADAR device.
Japanese engineers exploit their available patents and technologies to ensure an overall white light emission generated from a light conversion layer of semiconductor heterojunction. Light conversion layer is indispensible in optical devices for complementary colors, conical reflection and internal conical devices, optical lenses, and light guiding apparatus; however, some aspects of this technology remain to be improved. For example, when the heterojunction is placed on sapphire single crystal substrate, heat will be generated if the power is larger than 100 mini-watt, since sapphire has low heat conductivity (about 45 W/MK) and heat cannot be effectively dissipated from the chip of light-emitting diodes. Many methods have been exploited to overcome the drawback; dielectric SiC, for example, has heat conductivity three times or more of that of sapphire, leading to even higher power excitation. Such a device is first developed by an American company Cree; however, the cost is extremely high.
Semileds Company has engaged in resolving the issue of thermal stability of light-emitting diodes from its root cause by directly placing the semiconductor heterojunction onto copper substrate. Semileds's engineers put the semiconductor heterojunction made of In—Ga—N directly upon copper substrate. This approach employs a very special barrier to stop the diffusion between heterojunction and copper substrate.
Nevertheless, there are some drawbacks remained. Although the package of high-power light-emitting diodes has the characteristic of low heat resistance, the phosphor powder particles in light conversion layer are stilled heated. As a result, the luminescence quantum output of phosphor powder will decrease. For example, aluminum-yttrium garnet phosphor powder ((Y,Gd, Ce)3Al5O12), developed by the well-known Nichia Company has experienced a reduction of 25% for this parameter at T=373K. For this special series of phosphor powder, if Gd atom concentration ([Gd]) is increased to 0.5, this parameter will decrease by 50% at T=350K.